The Broad Brook Mill site, formerly known as the Millbrook Condominiums site, is located in the Broad Brook section of East Windsor, Hartford County, Connecticut. The property encompassed by the Broad Brook Mill site includes two separate lots, identified as Block 37, Lots 8 and 8A on East Windsor's Tax Assessor Map 22. This property is utilized for both commercial and residential purposes. Lot 8 (8.67 acres) is occupied by a former industrial mill building converted into a residential condominium building (21 units), two garage units, and a former boiler house. Lot 8A (1.93 acres) is occupied by a commercial complex, and a two-story brick office building.

The site is bordered approximately by a stream (Broad Brook) to the north, Main Street to the east, Mill Street to the south, and Broad Brook and 58 townhouses to the west. The townhouses are currently part of the Millbrook condominium association, but are not part of the site. Broad Brook flows west along the northern portion and south along the western portion of the site, respectively. The central area of the site is overgrown with wooded vegetation. The eastern portion of the site, along Main Street, slopes west and is covered by areas of bituminous pavement and grass. The western portion of the site is level. The southern portion of the site is bisected by Brookside Drive, slopes toward Mill Street and is covered with wooded vegetation.

The site was historically used for industrial purposes dating back to at least 1835 when a woolen mill was built by the Phelps Manufacturing Company, displacing earlier gristmills, sawmills, and a tannery. In 1849, the Broad Brook Company bought the mill and continued manufacturing woolen products until 1951. Manufacturing processes performed during this period included picking, carding, spinning, dressing, weaving, scouring, carbonizing, napping, shearing, and dyeing. In order to provide power for the woolen mill, the Broad Brook Company operated a coal gasification plant on a portion of the site, which resulted in the contamination of the site with polycyclic aromatic hydrocarbons (PAHs) as coal gasification by-products.

In 1954, United Aircraft Corporation, now United Technologies Corporation (UTC), purchased the site property and owned it until 1977, at which point UTC leased storage space at the site until 1981. From 1954 to 1967, at the site, UTC engaged in the manufacture of printed circuit boards, with the following associated processes: coil winding to produce low-voltage transformers; electroplating; chemical etching; photographic development; potting; soldering; assembly; and testing. Additionally, former mill buildings were used for the following: operation of a machine shop to produce small parts needed in the manufacturing process; a parts cleaner station, which utilized chlorinated solvents; a wastewater treatment plant to treat electroplating water; a paint spray booth to paint assemblies; a boiler house to provide steam and heat in the facility structures; a water treatment plant to provide quality water for manufacturing processes; and storage of surplus materials and equipment. From 1968 to 1974, UTC engaged in a boron filament manufacturing operation at the site. From 1974 to 1977, there was a boron filament manufacturing operation at the site, which was operated by Composite Materials Corporation, a subsidiary of Aluminum Company of America; Composite Technology Inc. continued manufacturing operations from 1977 to 1982.

In 1977, UTC conveyed the site property to Broad Brook Center, Inc., whose current existence is unknown. Broad Brook Center, Inc. leased building space to commercial and industrial users. In 1982, Broad Brook Center, Inc. conveyed the site property to James R. Testa and John Bartus, who in 1985, doing business as the partnership Broad Brook Center Associates, transferred title to the site property to Broad Brook Center Associates. In January 1986, the site property was sold to Connecticut Building Corporation. In May 1986, a fire destroyed many of the former mill buildings. In July 1986, Connecticut Building Corporation applied for a permit to develop 21 residential condominium units in the former mill building that had survived the fire. These residential condominiums were developed between 1990 and 1993.

Extensive surface and subsurface investigations have been performed on the Broad Brook Mill site. In August 1993, Heynen Teale Engineers collected soil gas, soil, and ground water samples. In October 1994, Connecticut Department of Environmental Protection (CTDEP) collected soil samples from 13 locations on the site. In October 1995, EMG collected soil and ground water samples. From December 1996 to October 1998, United Technologies Corporation and its wholly owned subsidiary Hamilton Sundstrand Corporation contracted Loureiro Engineering Associates, Inc. (LEA) to collect soil gas, soil, sediment, and ground water samples from the Broad Brook Mill site. Polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (solvents) are present in soils.

EPA initiated a Removal Site Investigation in December 1999, which included soil sampling, soil gas sampling, and an evaluation of indoor air. In April 2001, at EPA's request, United Technologies Corporation and its wholly owned subsidiary Hamilton Sundstrand Corporation agreed to perform voluntary removal activities at the Site. In May 2001, Loureiro Engineering Associates, Inc. began these activities, including installation of interim soil cover materials around the 21-unit condominium building, and asbestos abatement activities in the former boiler house. All activities were completed in July 2001.

EPA proposed this site to the National Priorities List (NPL) on December 1, 2000. The public comment for this ended on January 30, 2001. EPA and the Connecticut Department of Environmental Protection subsequently proposedto defer consideration of the site to the State of Connecticut for cleanup, rather than finalize its listing on the NPL. In December 2003, EPA, CT DEP, and Hamilton Sundstrand finalized certain agreements necessary to defer this site to the State. EPA and CT DEP entered into a Deferral Agreement, with the State taking the lead in ensuring cleanup of the site and EPA providing oversight. CT DEP entered into an enforceable Consent Order with Hamilton Sundstrand to develop and implement a cleanup plan. Additionally, EPA and Hamilton Sundstrand have entered into an Agreement for Recovery of Past and Future Response Costs, by which EPA will be reimbursed for its response costs.

The Deferral Agreement and Consent Order became effective on October 29, 2004, when Hamilton Sundstrand acquired ownership of the 21-unit mill building (formerly used as residential condominiums) and associated property from the Millbrook condominium association and individual unit owners. Residents are no longer living on the property and the site is fenced.

A draft Proposed Remedial Action Plan (RAP) was made available for public review at the Library Association of Warehouse Point in East Windsor, CT. The CT DEP accepted public comments on the Proposed RAP from July 22 through September 20, 2010, and held a public meeting and public hearing on August 30, 2010 at East Windsor Town Hall.

CT DEP approved the Remedial Action Plan (RAP) on November 30, 2010.

The Connecticut Department of Energy & Environmental Protection (CT DEEP, renamed from CT DEP) approved the Final Proposed Groundwater Remedy Work Plan and the Final Proposed Soil & Sediment Remedy Work Plan on September 8, 2011. Hamilton Sundstrand is currently working on design efforts and pursuing necessary permitting to begin work on site.

EPA has agreed not to pursue finalizing the proposed NPL listing at this time. If, however, cleanup cannot proceed under the deferral plan, EPA will restart efforts to finalize the proposed NPL listing with the intent of performing cleanup under the Superfund program.